I am guessing you are programming in python with pyserial installed. It seems that the readline() would return a empty line, and therefore its printing nulls. This is because of the timeout being passed with 1. The print statement adds a newline at the end, so it will keep printing newlines. This is why you would see your text momentarily.
You may want to use print(data, end="") if you don't want to add newlines at every print statement.
You may want to check if there is data on the returned readline() before passing to print(data), as it would keep printing empty strings.